def add_url(self, urls):
if url is None or len(url) == 0:
return
for url in urls:
if url not in self.new_urls and url not in self.old_urls:
self.new_urls.add(url)
输出可以用with语句来写：
with open('output.html', 'w') as f:
f.write("<html>")
f.write("<body>")
f.write("<table>")
